1. Field of the Invention
This invention relates to a process for production of a continuous plastic sheet by casting this plastic material or a reaction mixture of reactive components leading to this material in the liquid state on a mobile support comprising glass plates placed end to end and supported by a conveying means.
2. Description of the Related Art
Such a process is described, for example, in U.S. Pat. No. 4,136,214.
The material placed in the liquid state on the mobile bottom consisting of glass plates solidifies there by cooling, is separated from the plates downstream from the device for forming a continuous sheet, which then is wound and stored in this form, to be cut later into panels of dimensions suited to the use considered.
In practice, plastic sheets which are made by such a process are intended for the production of safety glass of the asymmetrical type, formed as a rigid, monolithic or laminated support comprising, on one of its faces, a transparent plastic layer, generally consisting of a polyurethane.
These glazings may be used in a windshield. There, the plastic layer, turned toward the inside of the vehicle, has various functions:
1. To protect the passengers from lacerations by pieces of glass in case of an accident;
2. If necessary, to act as an energy absorber in case of impact.
To meet these requirements, a sheet with two distinct plastic layers generally is used, which is assembled on a rigid, monolithic or laminated glass support, the energy-absorbing layer being applied against the glass, while the scratch-resistant layer is placed inside the vehicle.
It is clear that the plastic sheets used for this purpose should have excellent optical qualities and, for this purpose, they should be cast on a continuous glass ribbon, for example a glass ribbon which was just formed by floating. But this is hard to achieve in practice. Therefore, in practice a train of glass sheets of a constant thickness are placed edge to edge on a continuous conveyor and are used for a continuous bottom for casting of the reaction mixture. These glass sheets are used only as a support for the forming of the plastic sheet, and they generally are first coated with a separating agent intended to prevent adherence of the plastic to the glass and to facilitate the separation of the two after setting.
To protect the sections of the glass plates during this process, by preventing their mutual rubbing, and to prevent the penetration of the liquid plastic between the adjacent edges, it has been proposed to insert an elastic strip, for example, a foam strip, between the glass plates.
However, this solution presents the drawback that the cast plastic material changes the characteristics of the inserted foam strip so that if the glass plates are reused for a new cycle of casting, as is usually the case, it is necessary to replace the flexible elastic strips by new strips.
To prevent this change of the inserted strips, they may be protected by an adhesive ribbon glued to the adjacent edges of two successive plates and overlapping or covering the inserted strip. Unfortunately, this solution is not satisfactory, because the adhesive ribbon adheres firmly not only to the glass but also to the plastic which covers it and the subsequent pulling off of the sheet is made difficult, with the risk of tearing the latter.